Bolt Torque Explained
Eric Wiebrecht
Bolts
Here's an explanation of bolt tightening and how it applies to Buick engines, specifically the head-block joint:

Bolts are springs which are preloaded by turning them, clamping two or more objects together until their compression stiffness exceeds that of the tensile stiffness of the bolt. After this point, the bolt stretches like a spring and begins to apply clamp load to the joint.

Picture an engine power curve with a plateau in the peak region. X axis is RPM, Y axis is hp. Now change X axis into elongation (stretch), Y into load. This is now a good approximation of the stretch curve for a bolt.
Yield
Yielding, necking down, permanent elongation -- this is when the bolt is stretched past the spring state. The curve is altered, and after loosening, the bolt is longer than it originally was. The material had to come from somewhere, and, taffy-like, it does -- from the diameter (hence the term "necking down"). The next time the bolt is tightened, there is a new power curve to the right of the original one, a new X zero point, since the bolt is longer. Usually thread pitch will change (on the threads which were not engaged during tightening) so that when an unused bolt and a yielded bolt are put thread-to-thread, they don't exactly match.

The advantage of torquing bolts to their yield point is more consistent clamp load -- when a bolt is torqued, the object is to create enough clamping force to always keep the pieces together (like head and block). Pressure cycling (cylinder pressure) acts in the opposite direction, trying to push the two pieces apart. If the total bolt load in one area is ever lower than the separating force, the pieces will stretch the bolt(s) further and physically separate (ex: head gasket blowout). If the bolt load applied is always higher than cyclic separating force, then the joint is safe. If the bolt is tightened short of the "power peak", this cyclic load will cause the bolt force value to move up and down the slope of the "power curve", vs. a bolt torqued to yield where the force value is sliding across the plateau, varying clamping force less. In addition, the total load is higher, at the maximum possible when the bolt is at the yield point. There is a strange sensation felt during tightening when a bolt is yielding -- torque input stays constant (on that plateau of the "power curve") even during further tightening. Past this point, the clamp load will drop (past the "power peak") and the bolt will eventually fracture.
Torque Turn
Since bolts are springs, F=kX where F= force, k=spring rate, and X=stretch. Assuming that the bolt is clamping two infinitely stiff parts together with no slack between them, turning a 7/16 - 14 bolt (Buick head bolt) one turn will stretch it 1/14 of an inch. Multiply this by the spring rate of the bolt, and get the clamping force exerted. Bolts are pretty consistent spring rate-wise, so this is a good way to get a consistent clamping force -- turn the bolt a fixed amount once all of the slack (gasket squish, thread mesh slack, dirt, etc.) is removed. The initial torque step in torque-turn does this. The turning step afterwards is a consistent way to get the desired clamp load.
Why Not Just Torque?
Torque is feedback which is corrupted by a few factors. Torque, especially near the yield point, is greatly affected by friction under the bolt head and in the threads. Surprisingly, about 40% of the input torque goes towards under head friction, and another 40% to thread friction. This leaves 20% of the input torque for stretching the bolt. Adding oil, sealant, etc. to the threads or under the head usually reduces friction, so that for a given torque, an oiled bolt will stretch more (and thus will exert a higher clamping force) than a dry one. Also, a used bolt will have the threads and under head surfaces polished and will usually exert a higher load for the same torque. In both of these cases, the "power curve" slope would not be as steep as the one for a dry bolt.
Torque Sequence
Ideally a person would get 8 friends with torque wrenches and tighten all 8 head bolts in unison at the same rate (this is the way it is done at the factory via a multi-spindle unit). Since this is not practical, a sequence with multiple steps is used so that clamping force will be applied evenly and undue stress will not occur in either bolts or clamped parts.


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